A critical knowledge gap exists in the understanding of how medical marijuana (MM) policies have impacted emerging adults since MM became legal in California in 1996. It is unknown how MM has affected patterns of drug use, including alcohol, prescription, and illicit, among emerging adults. It is unknown how MM has impacted physical or psychological health among young medical marijuana patients (MMP). It is unknown how MM dispensaries influence health among young MMP, including the impact of closing MM dispensaries on access to MM. To address these gaps, a five-year project is being proposed in Los Angeles (LA) to study young MMP, young non-medical marijuana users (NMU), and MM dispensaries in natural settings using qualitative and quantitative methods. A long-term goal of the study is to recommend MM policies to local, state, and national officials that result in the most positive health outcomes for young adults and communities. The overall objective of this particular application is to determine the impact of LA's MM policies on emerging adults' physical and psychological health while understanding how medical marijuana dispensaries influence individual health. The central hypothesis guiding this study is that young MMP, as a result of access to MM and patient support offered by MM dispensaries, will report better physical and psychological health and less harmful patterns of drug use compared to NMU. This hypothesis is supported by preliminary data collected in LA on young MMP and NMU during a currently funded NIH study, which suggests important differences between these two groups as well as dissimilarities between young MMP and chronically ill, older MMP. The rationale for the proposed research is that young MMP and NMU will be impacted by MM policy longer than other adult cohorts with unknown consequences, while the diverse setting of LA - with its large number of MM dispensaries - is an optimal location to study this cohort. Guided by preliminary data, this hypothesis will be tested by pursuing three specific aims: 1) Determine the basis for MMP status and its impact on trajectories of physical and psychological health among emerging adults. 2) Determine the impact of MMP status on patterns of drug use among emerging adults, including intensity of marijuana use and misuse of alcohol, prescription drugs, and illicit drugs. 3) Describe the natural history of marijuana use in Los Angeles among MMP and NMU. The approach is innovative because it focuses on a critical cohort of young adults, it builds upon findings and theoretical perspectives from the field of non- medical marijuana research, and it integrates quantitative and qualitative research methodologies into a longitudinal research design. The proposed research is significant since study data can provide evidence- based rationales for modifying patient health behaviors, physician guidelines, and city ordinances. Ultimately, determining the impact of medical marijuana on the health of emerging adults and communities in LA has the potential to inform medical marijuana policy at the local, state, and national levels.